Deposition-precipitation with titania

Figure 1. TEM image of a titania supported gold catalyst (1.7wt.% Au) prepared by deposition-precipitation (gold particle size = 5.3+ 0.3 nm, dispersion = 36%). (Reprinted from Reference [84], 2000, with permission from American Chemical Society).

The success of Haruta s early work lay in his choice of preparation method and support. Gold particles of the necessary small size were first obtained by coprecipitation (COPPT) and later by deposition-precipitation (DP) (see Sections 4.2.2 and 4.2.3) classical impregnation with HAuCLj does not work. The choice of support is also critical transition metal oxides such as ferric oxide and titania work well, whereas the more commonly used supports, such as silica and alumina, do not work well or only less efficiently. This strongly suggests that the support is in some manner involved in the reaction. 

Wet impregnation is not feasible for all catalyst types. For instance, vanadia on silica cannot be prepared by means of wet impregnation. With homogeneous deposition precipitation (HDP), however, it is possible to prepare vanadia on silica catalysts [21], The principle of this method is to use a lower valence of the metal, which may be produced by cathodic reduction of the respective metal ion. The reason for using lower valence state metal ions is the lower acidity compared to that of the higher valence state and the higher solubility of the metal ions. This technique was used for the preparation of silica-supported vanadia, titania, and molyb-dena catalysts [22, 23]. 

Titania/silica catalysts were prepared by a conventional procedure (precipitation) and a complexing-agent assisted sol-gel method. The effect of preparation methods of titania/silica catalysts on their properties and catalytic activities in the oxidation of olefins were examined. The sol-gel method gave the best dispersion of titania. In contrast, using the precipitation method, titania is deposited at the external surface of silica with formation of crystalline particles. The sol-gel catalysts are more effective for epoxidation of olefins because of the high dispersion of Ti in them. 

In the adsorption study with titania (33), the levels of silica used were such that dispersions were always above the solubility limit for amorphous silica. Although the final concentration of aqueous silica in a typical DS preparation procedure might be between 10-2 and 10 1 mol/dm3 and hence close to or above the solubility limit, the DS procedure certainly involves exposure of titania surfaces to soluble silica species prior to precipitation. Therefore, the value of the previous adsorption study (33) for understanding the details of DS deposition is limited. The final silica concentration reached depends on the levels of titania used and the degree of coating desired. Such final concentrations refer to added silica—in fact bulk precipitation may never occur even if the solubility edge of Figures 5 or 6 is traversed (27). 

The deposition-precipitation method is applied to pure anatase titanias with high surface areas and silica mesostructures doped with Ti/Ti02. The effect of the type and density of surface functionalities both on the deposition of gold and on the catalytic properties of the resulting materials in the oxidation of CO in the presence of H2 are discussed. 

Deposition-precipitation. The samples were synthesized accordingly to the method reported in literature [2, 6]. Aqueous solutions of the metal (10 M) were prepared by dissolving HaPtCle.hHaO (Acros) or (NH4)2PtCl4 (Acros) in distilled water. An amount of Ti02, chosen to obtain a desired metal loading of 3 wt%, was dispersed in these solutions. The pH of the titania dispersion was then adjusted to 10 with 0.1 M NaOH. The deposition reactions were carried out by stirring the solution for 2 h or 24 h at room temperature or 60°C (exact conditions are specified in the text). The products were washed 4 times with distilled water to remove chloride and sodium ions and then dried in a vacuum oven at 60°C for 16 h. The precursors were then heated at 300°C for 4 h under a flow of Hz in He (10/90). 

SBA-15 is a mesoporous ordered silica with promising properties as a catalyst support [1], where functionalization of the siliceous carrier with an active component can be achieved through a number of methods [2]. In this study we describe the use of the deposition-precipitation technique (DP) employed to functionalize the SBA-15 with Pt. The use of the DP method on the SBA class of materials has not been studied so far, excluding a few studies dealing with deposition of gold on titania-modified SBA-15 [3, 4]. The method... 

Ceria was loaded on mesoporous titania by deposition precipitation (DP) method. Before deposition, the mesoporous material was suspended in water by ultrasound technique. Ceria was deposited by precipitation of Ce(N03)3 6H2O with Na2C03 at 60 °C and pH 9.0. Analytical grade chemicals were used in the support preparation. The precipitate was aged in a course of 1 h at the same temperature, filtered and washed carefully until absence of N03 ions. The sample was dried under vacuum at 80 °C and calcined in air at 400 °C for 2 h. Ceria modifying additive is 20 wt.%. The sample was labeled as CeMTi. 

This method of deposition-precipitation is probably the most used for the preparation of gold catalysts since it readily leads to the formation of small gold particles (2-3 nm). This method was first proposed by Haruta et al. [25, 26]. The pH of the solution containing HAuCh and the oxide support is adjusted by addition of NaOH, often 7-8 for titania or alumina. The suspension is stirred for 1 hour at 70-80°C. The catalyst is then washed with water to eliminate as much chloride and sodium ions as possible, dried between RT and 100°C, then usually calcined in air. 

FIGURE 14.8 Deposition-precipitation of gold on titania with NaOH for various pH (a) An loading (b) gold particle size from Reference 32. 

The decrease in the gold particle size with the time of deposition-precipitation (Table 14.5) was attributed to a phenomenon of peptization (redispersion), as observed, for instance, when nanocolloids of titania, formed by hydrolysis of alkox-ides, agglomerate rapidly to produce large precipitates, but can be slowly redispersed through the action of nitric acid. 

Au/Ce-Ti-O materials have been scarcely used in catalysis [1-5]. In the few available reports on the synthesis of this kind of materials, the Ce-Ti-O supports were synthesized by a sol-gel method [1,2,4,5] or by incipient wetness impregnation of aqrreorrs solution of cerimn nitrate on titania [3]. In these works, Au was loaded by Deposition-Precipitation [1-5]. We have previously synthesised Ce-Ti-O supports with different Ce/Ti molar ratios by solvothermolysis [6]. In the preserrt work, Ce-Ti-O supports, with higher Ce contents, were loaded with Au by a double impregnation method (DIM) [7], characterised by several techniques and tested for CO oxidatioa To the best of our knowledge, this is the first report with the combination of the solvothermal method for Ce-Ti-O synthesis with the DIM approaeh for gold loading. 

It was concluded that gold particle-support interaction is required together with careful selection of the titania-silica support and control of the gold-particle size. The use of pH 7.0 in the deposition-precipitation method was also recommended, together with calcination at 573 K. Propene oxide has recently been obtained via a two-stage process involving dehydrogenation of propane to propene (selectivity of 57%), followed by a propene to propene oxide oxidation with a selectivity of 8%, and... 

F-T Catalysts The patent literature is replete with recipes for the production of F-T catalysts, with most formulations being based on iron, cobalt, or ruthenium, typically with the addition of some pro-moter(s). Nickel is sometimes listed as a F-T catalyst, but nickel has too much hydrogenation activity and produces mainly methane. In practice, because of the cost of ruthenium, commercial plants use either cobalt-based or iron-based catalysts. Cobalt is usually deposited on a refractory oxide support, such as alumina, silica, titania, or zirconia. Iron is typically not supported and may be prepared by precipitation. 

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